Equine Protozoal Myeloencephalitis is a very rare brain disease that is associated with protozoa that causes neurological disease. Historically, horses with this disease had a poor prognosis. It was unexpected that horses could resolve the clinical signs of EPM because it was thought that parasites caused physical damage in the brain or spinal cord. It turns out that there are three diseases masquerading as EPM, the good news is that these diseases are common and can be identified and treated.
Our view is different because we identify the cause and consequences of infection. Infection causes inflammation of the nerves in the body and central nervous system. Protozoal-induced inflammation can be effectively managed and treated. In some cases protozoal disease can be prevented.
Causes and risk factors of EPM Sarcocystis neurona is a parasitic protozoa that was isolated from horses with EPM. Another organism, Neospora hughesii is a very rare cause of EPM. Studies using postmortem data show most EPM horses were young Thoroughbreds, Standardbreds, and Quarter Horses.
Causes and risk factors of Sarcocystosis If you look at seroprevalence data (a measure of antibodies in horses) almost all horses are positive for S. neurona because they had exposure to the organism and quickly resolved this very common infection. Most horses do not get EPM and most horses resolved sarcocystosis. Horses can get clinical signs associated with inflammation from diseases that look like EPM.
Causes and risk factors of polyneuritis Polyneuritis is a generalized neuromuscular disease that can be caused by S neurona, S fayeri, or other factors (vaccination reaction, bacterial infections, viral infections, Lyme disease). Almost 1/3 of horses in this country have muscle cysts that are the end stage infection of Sarcocystis fayeri. Usually this infection is benign, unless the horse is debilitated. However, as the cysts breakdown they can release substances that induces an inflammatory response. These horses have “sub-clinical” disease and become clinically ill as the inflammatory cycle isn't turned off. The cycle of inflammation produces signals that keep the inflammation going, in turn producing more pro-inflammatory signals.
Diagnostic testing There are no EPM tests! There are antibody tests to detect S neurona and other organisms associated with EPM, such as S. fayeri. All “EPM” tests in the live animal are different methods of detecting antibody against S. neurona. Antibody tests based on S. neurona have been used to predict how likely it is that a horse may have EPM. The problem is that antibody lingers after S. neurona is gone. And the level of antibody reached during and after infection is not statistically related to infection. S neurona tests are useful to rule in or rule out S. neurona as the cause of disease, but is not useful to detect current infection. Tests for S. neurona are specific (SAG 1, 5, 6) or non-specific (SAG 2, 4/3). Non-specific tests use carefully diluted serum to remove cross-reactive antibodies such as those that detect S. fayeri. Sarcocystis fayeri tests detect antibodies directed against disease causing proteins and are associated with disease.
Inflammation causes clinical signs during infection. The inflammatory component of disease causes inflammation of the nerves, called neuritis. Inflammation can become the issue (polyneuritis equi) and the inflammatory cycles causes chronic disease. Uncontrolled chronic inflammation can be life-threatening and is recognized as cauda equina syndrome.
A panel of tests that include the likely causes of neuromuscular diseases are SAG 1, 5, 6, S fayeri, MP2/MPP, Lyme, and CRP levels.
Treatment There are several approaches to treatment. Each animal is different and responds differently to treatment, however if clinical signs aren’t improving after initiation of treatment a review of the test results, further testing, and a change in treatment are indicated. Double dosing an ineffective treatment is not a good treatment plan. If the disease process is inflammation, it is important to monitor the inflammation using CRP testing, treat until the inflammatory process has resolved (often weeks after the resolution of clinical signs), and then prevent relapses by monitoring the level of inflammation.
Prevention If it is determined that a horse is continually exposed to S neurona or S fayeri preventing infection is possible. Preventing the production of antibody to S neurona with some drugs has been proven ineffective for preventing disease. It is important to test and determine if there is continued exposure to the parasite because overuse of antimicrobials is unwise. Preventing pathologic inflammation is best accomplished by monitoring the CRP levels.